COPII is that it can form typical spherical vesicles and tubules to transport large molecules like collagen precursors, which cannot fit inside typical spherical vesicles. COPII structure has been discussed in an open article and will not be a focus of this article. These are examples of the much larger set of cargo adaptors. Evolutionary considerations The most recent common ancestor (MRCA) of the eukaryotes must have had a mechanism for trafficking molecules between its endomembranes and organelles, and the likely identity of the adaptor complex involved has been reported. It is believed that the MRCA had 3 proteins involved in trafficking and that they formed a heterotrimer. That heterotrimer next "dimerized" to form a 6 membered-complex. The individual components further changed into the current complexes, in the order shown, with AP1 and AP2 being the last to diverge. In addition, one component of TSET, a muniscin also known as the TCUP protein, appears to have evolved into part of the proteins of opisthokonts (animals and fungi). Parts of the AP complexes have evolved into parts of the GGA and stonin proteins. There is evidence indicating that parts of the nuclear pore complex and COPII may be evolutionarily related. Formation of transport vesicles The best characterized type of vesicle is the clathrin coated vesicle (CCV). The formation of a COPII vesicle at the endoplasmic reticulum and its transport to the Golgi body. The involvement of the heterotetramer of COPI is similar to that of the AP/clathrin situation, but the coat of COPI is not closely related to the coats of either CCVs or COPII vesicles. AP-5 is associated with 2 proteins, SPG11 and SPG15, which have some structural similarity to clathrin, and may form the coat around the AP-5 complex, but the ultrastructure of that coat is not known. The coat of AP-4 is unknown. An almost universal feature of coat assembly is the recruitment of the various adaptor complexes to the "donor" membrane by the protein Arf1. The one known exception is AP-2, which is recruited by a particular plasma membrane lipid. Another almost universal feature of coat assembly is that the adaptors are recruited first, and they then recruit the coats. The exception is COPI, in which the 7 proteins are recruited to the membrane as a heptamer. As illustrated in the accompanying image, the production of a coated vesicle is not instantaneous, and a considerable fraction of